With lowered flags, tolling bells and solemn vigils, America today pauses to reflect on the moment when the crack of a rifle pierced a sunny Texas afternoon — and the heart of a nation.

In Dallas, a moment of silence will fill Dealey Plaza, where shots rang out at 12:30 p.m. on Nov. 22, 1963, cutting down a vibrant President John F. Kennedy as he waved to an adoring crowd from his motorcade.

“His death forever changed our city, as well as the world,” Dallas Mayor Mike Rawlings said. “We want to mark this tragic day by remembering a great president with the sense of dignity and history he deserves.”

In a nod to Kennedy’s military service, there will be an Air Force flyover, and the 73-member US Naval Academy Men’s Glee Club will sing “The Battle Hymn of the Republic.”

Religious leaders will offer prayers, and historian David McCullough will read parts of JFK’s presidential speeches. Parkland Memorial Hospital, where he Kennedy was declared dead, will hold a quiet memorial.

Just 5,000 tickets were given out for the ceremony at the plaza in the shadow of the Texas School Book Depository building, where 50 years ago, Lee Harvey Oswald lay in wait for the president.

Conspiracy theorists, who each year converge on the area, have been given their own place to gather, away from Dealey Plaza.

In Boston, the Massachusetts-born Kennedy will be honored at a private event at the JFK Library and Museum.

The library, which usually doesn’t observe the anniversary, will launch an exhibit of never-before-seen artifacts from Kennedy’s funeral.

Singer James Taylor, a Navy choir and saxophonist Paul Winter, who played at JFK’s White House, will perform, and Massachusetts Gov. Deval Patrick will read from JFK speeches.

“We want our tone to be respectful, and we want it to have a certain reverence,” said library executive director Thomas Putnam. “But we also want it to be hopeful and end on this notion of what JFK stood for.”

On Cape Cod, near the Kennedy compound in Hyannis Port, storefronts will be draped in black bunting.

And in DC, President Obama will host a private reception with members of the Peace Corps, the organization Kennedy established in 1961.

There will be a memorial Mass at DC’s Cathedral of St. Matthew the Apostle, where JFK’s funeral was held. Large crowds are expected to pay homage at Arlington Cemetery, where the Eternal Flame was recently restored at Kennedy’s grave.

The Newseum in DC will host a JFK Remembrance Day that includes a rebroadcast of CBS News’ live TV coverage of the shooting.

Meanwhile, cities will fly flags at half-staff, with a moment of silence at 1 p.m. (CST, or 2 p.m. in New York), the time JFK was pronounced dead.

High-profile New Yorkers recalled one of the country’s darkest days.

Ex-Yankee slugger Reggie Jackson was a Pennsylvania high-school senior when the announcement came over the school PA system.

“I remember going to my locker with tears in my eyes,” he said. “I went home, and my dad was there . . . My dad said it was more of a loss for the colored people — that was the term then — because this guy was going to live by the Constitution. Kennedy was a friend of the colored people. You felt that as a young, colored kid.’’

Timothy Cardinal Dolan was an eighth-grader at Holy Infant school in Ballwin, Mo., and running a lunchtime bake sale.

“I remember it was a dreary, rainy day anyway, so we couldn’t go outside for recess. Kids were coming by to buy cupcakes,” he said, when a student who had gone home for lunch came back with the news.

“We had a TV in our room . . . and we saw the famous announcement that President Kennedy was dead.”

“It still chokes me up. We were all devastated. The whole school went to church.”